1. Field of the Invention
The present invention relates to a communications system and method, particularly, to a communicaitons protocol for the detection of speech transmissions amid control signals, and, more particularly, to an improved system and method for distinguishing valid speech frame transmissions from control signals and random radio/frequency (RF) noise, thereby avoiding speech quality degradation by minimizing the chance of incorrectly processing a non-speech frame as if it were speech.
2. Background and Objects of the Invention
The evolution of wireless communication over the past century, since Guglielmo Marconi's 1897 demonstration of radio's ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi's discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which time the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. The exponential growth of mobile telephony will continue in the coming decades, as this wireless network interacts with and eventually overtakes the existing wireline networks.
The Global System for Mobile (GSM) communications is a second generation cellular system standard developed to solve various fragmentation problems of the first cellular systems in Europe. GSM is the world's first cellular system to specify digital modulation and network level architectures and services. Currently, GSM is the most popular standard for new radio and personal communications equipment throughout the world.
The Satellite Air Interface Specification (SAIS) is essentially an adaption of the GSM specification and is designed to provide telephone coverage by use of a geostationary satellite, such as the Asia Cellular Satellite (ACeS) to be deployed over SouthEast Asia. The ACeS system is designed to provide coverage to areas having limited land-line and cellular infrastructures, allowing the use of hand-held pocket phones throughout much of SouthEast Asia. Although the SAIS attempts to adhere to the GSM standard, there are differences between the two systems, such as the introduction of a high-margin channel to reach disadvantaged or remote users and a lower speech coder rate. In contrast to the GSM full-rate speech of 13 Kbps, the SAIS vocoder codes speech at a rate of 3.6 Kbps. This leads to the SAIS ability to support 32 users per 200 KHz effective channel versus GSM's 8 users for the same bandwidth.
In GSM, all associated and common control channel signaling formats are defined on the basis of fixed length messages that undergo extensive error control coding prior to transmission over the airlink. For example, a fixed length message is block encoded using a conventional fire code and then convolutionally encoded. The resultant encoded message is then interleaved and sent over the airlink.
Through one of the Associated Control Channels, the Fast Associated Control Channel (FACCH), control signals are sent in-band with speech frame transmissions. FACCH provides quick communication between a base station and a cellular phone for purposes such as hand-overs between cells. However, FACCH messages steal or blank out speech frames while the necessary handover or other signaling information is transmitted. Accordingly, a receiving device must distinguish incoming signals as either speech frames or control data. As is understood in the art, FACCH and other control signals generally cause a speech decoder to repeat a previous speech segment or mute. Where a FACCH signal is mistakenly interpreted as a valid speech frame and passed through the speech decoder, the FACCH signal may introduce loud pops or other artifacts into the audio path. Thus, without adequate safeguards to prevent such a misinterpretation, the receiving user will perceive degraded speech quality due to the corrupted audio path.
In addition, a common feature in satellite and terrestrial digital cellular standards is Discontinuous Transmission (DTX). This feature allows a transmitter to save power and reduce RF interference by transmitting at a reduced duty cycle during periods when there is no voice activity. During a DTX period, voice may restart at any time prompting the transmitter to resume normal transmission. Accordingly, the receiving radio must always be ready to receive speech. This implies that the receiver remains on during DTX periods searching for a valid speech frame. There is a chance that the random noise on the air will occasionally pass through the receiver and be interpreted as a valid speech frame which gets played. Without some corrective action (as described in this disclosure), the mathematical probability of a noise frame passing into the audio path during a DTX period is quite significant. If a frame of random noise does mistakenly get passed to the speech decoder and played, it will likely create a pop or other audio artifact within the DTX period, thereby degrading the perceived audio quality.
In an effort to prevent the aforementioned sources of audio degradation, current digital standards have some reasonably straightforward and robust methods for distinguishing speech and FACCH signals. Also, DTX periods are currently distinguished by using the quality of a Viterbi metric or the strength of sync correlation, as is understood in the art. The problem is that the SAIS is presently inadequate to prevent these sources of audio degradation.
Accordingly, it is an object of the present invention to prevent the interpretation of FACCH or other overriding control messages as speech, thereby avoiding artifacts that degrade speech quality.
It is another object of the present invention to avoid the conversion of random noise into speech frames during DTX periods.